One of Ms. Aggarwal’s areas of expertise is developing a clean energy standard, which is the percentage of non-fossil fuels that utilities must achieve in their electricity generation and sale. By setting a standard without citing the source, the guideline aims to enable businesses and utilities to identify the most efficient way to achieve the goals. With the Senate now under Democratic control, the possibility of getting such a mandate passed could be within our grasp.

Maggie Thomas, who served as climate advisor in the presidential campaigns of Washington Governor Jay Inslee and Massachusetts Senator Elizabeth Warren, will serve as Ms. McCarthy’s chief of staff.

Jahi Wise, political director of the Coalition for Green Capital, a nonprofit group committed to investing in clean energy, will be a senior advisor on climate and finance.

The White House’s expansive team, which is not subject to Senate approval, has caused consternation among Republicans. Wyoming Senator John Barrasso, who will be the top Republican on the Senate Energy Committee, said he believed that stepping up energy innovation rather than “appointing countless unchecked tsars” would be best for both the economy and the environment.

James P. Pfiffner, Professor Emeritus of Public Policy at George Mason University and an expert on the Presidency, noted that Presidents have increasingly centralized control in the White House, creating special positions on high priority policies with mixed results to have.

A new White House climate office with at least five employees was a lot, he said, and a White House “Tsar” like Ms. McCarthy would have its challenges.

“White House staff have no power to make decisions about spending or staff,” he said. “Of course they can be powerful, but only to the extent that their policy area is of paramount importance to the President.”

Christopher Flavelle contributed to the coverage.